Last week when just running around, the engine sounded "noisy" - not exhaust noise, just sounded like it was laboring. This was when I was running at about 3000 RPM's with a 23 Mirage Plus on a 1.36:1 drive. I was testing out this prop, and it was a step up from the Mirage 21 that was on there before, so maybe I was lugging the motor a little bit. Keep in mind that it was about 103* ambient air temp at the time, so that may have been a factor.

Fast forward to last night - I decided to pull the plugs and run a compression test. This engine was installed last year, and now has around 40 hours on it, so it should be pretty well broken in. Plugs were black and sooty, which I suspected based on the fact that the engine was running rich at idle. I will need to address this issue later, but I wanted to err on the rich side to avoid melting down a new engine. The real eye opener to me was the compression numbers. With two trips around per cylinder, the gauge was reading about 185 PSI, and it would approach 200 PSI with a third trip around. That sounds high to me. Readings were consistent for all 8 holes, so I guess that is a good thing. No excessive oil consumption with this engine.

Specs on this engine: 489 CI - .030 over 4.25 bore X 4.25 stroke, flat top pistons, 088 heads with 121 CC chambers. Compression with a .045 gasket should come out to around 8.7:1. Cam specs are in the ball park of: intake - 224* @ .050, .575 life, exh - 236* @ .050, .578 lift, 114* LSA. Could it be the short overlap of the cam that is producing such a high dynamic CR? I am a little bit worried about what gas to run in it now. I thought with this relatively low CR I would be good to run 89 in it occasionally, now I am not so sure.

Isn't the HP 500 EFI cam running similar spec's? If so, I would assume these engines should pull similar numbers on a compression test. Does anyone have any real world numbers pertaining to this?

I am scratching my head a little over this...

Just from experience, double check the reading with another gauge or compression tester

2 years ago I was getting really high numbers on my snowmobile engine, I was WTF?!?!?!?
I was also getting 180-200 psi!!!! Come down to it the gauge was WAY OFF. I barrowed my neighbors snap on compression tester.... 135-140 psi. right where it should be.

NSX, that thought occurred to me too, but I used this same gauge on my 454 that was in there before, and I was getting 165 PSI on that engine with the same gauge. That motor had 9.5:1 static CR, but the cam was on a 112* LSA.

Looks great to me!! Run 91-93 and let her eat!!!

I'll probably put my 24 module back in. I have an HP 500 module in there now, which I think is 20* or 22* advance. That would allow me to dial back my initial advance a little bit and take some load off of the starter. Which is a whole nother story on a different thread.

20* or the 22* will not hurt the starter,,that would be the intinal timing that hurts it..thease modules come in between 2500 and 3500 rpm's. if you haver a mini dome on your piston then i can believe the compression readings..on mine i get 185 psi on all 8 cylinders..i run my timing @ 34* on 93 premimum fuel..12 yrs and no hick ups and runs like a raped ape..if you have alunimum heads i would not worrie about it,,espically with black sparkplugs..

With the smaller module, it will require me to run more initial timing in order to get to 34* total timing. For example, I would have to run 14* initial timing with a 20* module. This usually makes it tougher to turn over the engine, in my experience.

No domes at all on the pistons - Mahle flat tops, with valve reliefs on the intake side. At this point think it has more to do with the cam timing.

HP500 modules have 20* of advance all in by 3000rpms, so you should be running 12-16* base timing.
Reducing the base timing 4* will do nothing. Plenty of people run 32-34* locked timing without issues.